4-Aminodiphenylamine (4-ADPA) is an intermediate for making paraphenylenediamines (PPDs) which are antidegradants for various polymers including rubbers. 4-ADPA can be produced in various ways. First, 4-ADPA can be produced by reacting p-chloronitrobenzene with an aniline derivative in the presence of an acid acceptor to produce 4-nitrophenylamine, followed by the reduction of the nitro group. See, for examples, U.S. Pat. Nos. 4,187,248 and 4,683,332. Second, 4-ADPA can be produced by the hydrogenation of p-nitrodiphenylhydroxylamine. See, for examples, U.S. Pat. Nos. 4,178,315 and 4,404,401. Third, 4-ADPA can be produced by the head-to-tail coupling of aniline. See, for example, U.S. Pat. No. 4,760,186. Fourth, 4-ADPA can be produced by reacting acetanilide and nitrobenzene in DMSO to make nitrosodiphenylamine, followed by the reduction of the nitrosodiphenylamine. Fifth, 4-ADPA can be produced by a one-step reaction in which nitrobenzene is contacted with hydrogen and react with aniline in the presence of a hydrogenation catalyst, a hydrogenation inhibitor, and an acid catalyst. Sixth and currently the preferred reaction route for the commercial production of 4-ADPA, aniline and nitrobenzene are condensed to produce 4-nitrosodiphenylamine (4-NODPA) and 4-nitrodiphenylamine (4-NDPA) which are then hydrogenated to produce 4-ADPA. See, for examples, U.S. Pat. Nos. 5,117,063 and 5,453,541.
In the currently preferred process, the condensation reaction of nitrobenzene and aniline to produce 4-NOPDA and 4-NDPA are conducted in the presence of the phase transfer catalyst, typically tetramethyl ammonium hydroxide (TMAH), which is also used as an organic base. In the process, a small amount of azobenzene, phenazine, and other by-products are produced. 4-NDPA and 4-NODPA are then catalytically hydrogenated to produce 4-ADPA.
The current process requires a large amount of the organic base in the aqueous solution as the catalyst. The catalyst may be closely bound to the reaction products, 4-NDPA and 4-NODPA, after the condensation reaction, and thus, cannot be separated from the reaction products and regenerated in situ. The catalyst can only be released after the 4-NDPA and 4-NODPA have been hydrogenated to 4-ADPA, therefore, must go through the hydrogenation reaction. The catalyst is somewhat unstable, and often decomposes during the hydrogenation and subsequent concentration and recycle steps. Higher temperature, longer reaction time, and larger amount used lead to even greater decomposition of the catalyst.
The current process for producing 4-ADPA using water-soluble phase transfer catalysts also consumes a large amount of energy to protect and recycle the catalyst. The condensation of aniline and nitrobenzene requires low water content. While the organic base catalyst utilized in the condensation reaction can be extracted after the hydrogenation reaction, the concentration of the catalyst in the water phase extracted is low. It is even lower in the reaction system after the addition of methanol for separating the organic and aqueous phases. In order to recycle and reuse the organic base catalyst, it must be concentrated, which requires the use of additional energy.
Furthermore, the current production process for producing 4-ADPA from aniline and nitrobenzene may be unstable. Impurities are formed due to the continuous decomposition and reaction of the condensation catalyst during the subsequent steps before the catalyst can be recycled, which reduce efficiency and impede production. The reaction conditions are continuously changing as these impurities mount thereby altering the reaction conditions for condensation, hydrogenation, and especially phase separation. Thus, the process of producing 4-ADPA becomes less predictable and controllable.
The current process for producing 4-ADPA requires stringent conditions for performing the hydrogenation reaction which ultimately slows the production. For example, in order to prevent the condensation catalyst from decomposing during the hydrogenation reaction, the temperature of the hydrogenation reaction must be limited to 90° C. or lower. As a result, a hydrogenation catalyst with high activity at low temperature must be used, usually a noble metal catalyst. Noble metal catalysts are expensive and often require an organic solvent to accelerate the reaction. Such solvents will ultimately need to be recovered from the reaction system thereby increasing energy costs.
U.S. Pat. No. 6,395,933 describes a process for making 4-ADPA by reacting nitrobenzene and substituted aniline at a controlled temperature in the presence of a strong base and a phase transfer catalyst. The process results in low yields and increased side reactions. The process is costly and also requires an oxidizing agent which makes it unsuitable for the commercial production.
U.S. Pat. No. 6,495,723 describes a composition for use in the condensation of aniline and nitrobenzene which is composed of a solid carrier, typically zeolite, having interior channels containing a base. The cross-sectional dimensions of the channels provide an environment that improves the selectivity of the reaction such that undesired by-products such as phenazine or azobenzene are limited. The internal diameter of the zeolite carrier described in the patent is quite small such that the interior channels of that carrier are quite restrictive. Because of the limited utilization of interior surface, any attempted regeneration reaction of the organic catalyst would be mainly carried out on exterior surface. Furthermore, the small diameter of the zeolite internal channels prevents high loading values for the organic catalyst. As such, more catalyst would need to be added to a condensation reaction in order to maintain high catalytic activity and industrial value.
U.S. Patent Publication No. 2009/0048465 describes a complex base catalyst comprised of tetraalkyl ammonium hydroxide, an alkali metal hydroxide or oxide, and a tetraalkyl ammonium salt in the aqueous form that reduces the need to tightly control the quantity of protic materials in the condensation reaction. The complex base catalyst also decreases the conversion of tetraalkyl ammonium hydroxide to tetraalkyl ammonium carbonate thereby reducing the need to replenish the catalyst during the reaction. However, the complex base catalyst is not in a solid phase and therefore still must be separated, regenerated, and recycled.
Thus, in the condensation reaction of aniline and nitrobenzene, the current process for producing 4-ADPA using organic base catalyst requires a large amount of the catalyst and need to recycle the catalyst after several reaction steps. The current process can not be completed rapidly, and can also consume high energy. The current process requires increased solvent usage and more solvent recycle steps are needed, thus, the impurities will increase which lead to the decrease in efficiency and quality of the 4-ADPA production process. Therefore, there is a need to overcome the disadvantages of the current process for producing 4-ADPA.